A typical gas turbine engine includes one or more compressors, a combustor, and one or more turbines each connected by a shaft to an associated compressor. In most modern engines the combustor is an annular combustor in which a radially inner liner and a radially outer liner cooperate with each other to define an annular combustion chamber. During operation, a high temperature stream of gaseous combustion products flows through the combustion chamber. Because of the high temperatures, the liner surfaces that face the hot gases are susceptible to damage. It is, therefore, customary to protect those surfaces with a film of coolant, a protective coating, a heatshield, or some combination thereof.
One type of combustor is referred to as a thermally decoupled combustor; one type of thermally decoupled combustor is referred to as an impingement film cooled combustor. In an annular, impingement film cooled combustor, the inner and outer liners each comprise a support shell and a set of temperature tolerant heatshield panels secured to the shell to protect the shell from the hot combustion gases. A typical heatshield panel has a shield portion whose platform is rectangular or approximately rectangular. When secured to the shell, the shield is oriented substantially parallel to the shell so that one side of the heatshield, referred to as the hot side, faces the hot combustion gases and the other side, referred to as the cold side, faces toward the support shell. One or more threaded studs project from the cold side of each shield. In a fully assembled combustor, the studs penetrate through openings in the shell. Nuts threaded onto the studs attach the heatshield panels to the shell.
A principal advantage of a thermally decoupled combustor is that the heatshield panels can thermally expand and contract independently of each other. This thermal independence improves combustor durability by reducing thermally induced stresses. Examples of impingement film cooled, thermally decoupled combustors may be found in U.S. Pat. Nos. 6,701,714 and 6,606,861.
Various types of projections other than the studs also extend radially toward the shell from the cold side of each shield. These projections, unlike the studs, are not intended to penetrate through the support shell. One example of a non-penetrating projection is a boundary wall extending around the cold side of the shield at or near the shield perimeter. A typical boundary wall has an origin at the shield portion of the heatshield and a terminus remote from the shield. The height of the wall is the distance from the origin to the terminus. The terminus contacts the support shell thereby spacing the shield portion from the shell and defining a substantially sealed, radially narrow coolant chamber between the shell and the cold side of the shield. Alternatively, the height of the wall may be foreshortened over part or all of its length resulting in interrupted contact, or the absence of contact, between the wall terminus and the shell.
An impingement film cooled combustor liner also features numerous impingement holes that perforate the support shell and numerous film holes that perforate the heatshield panels. The impingement holes discharge a coolant (usually cool air extracted from the engine compressor) into the coolant chamber at high velocity so that the cooling air impinges on the cold side of the heatshield panel to help cool the heatshield. The impinged cooling air then flows through the film holes and forms a coolant film along the hot side of the heatshield.
In a state of the art impingement film cooled combustor, both the support shell and the heatshield panels are made of nickel alloys, although not necessarily the same alloy. In more advanced impingement film cooled combustors, the shell may be made of a nickel alloy and the heatshield panels may be made of a refractory material. Refractory materials include, but are not limited to, molybdenum alloys, ceramics, niobium alloys and metal intermetallic composites.
Despite the advantages of thermally decoupled, impingement film cooled combustors, they are not without certain limitations. For example, it may become apparent during engine development testing, or as a result of field experience, that it would be advisable to divert some of the coolant that would otherwise flow through the film holes in order to use that coolant for other purposes. This could be accomplished by radially foreshortening at least a part of the boundary wall that projects from the cold side of the heatshield panel, thus achieving the desired diversion of coolant from the coolant chamber. Alternatively, product development tests or field experience may suggest the desirability of radially lengthening a foreshortened boundary wall in order to reduce or curtail coolant diversion. These changes can be effected by modifying the tooling used to manufacture the heatshield and/or by revising the specifications that govern heatshield finishing operations such as machining. However introducing such changes can be expensive and complicated for the-engine manufacturer.
Additional limitations might affect advanced combustors that use a nickel alloy support shell and a refractory heatshield, especially at the interface where a heatshield boundary wall or other non-penetrating projection contacts the support shell. Because the refractory heatshield panels are intended to operate at higher temperatures than nickel alloy heatshields, considerable heat can be transferred across the interface where the heatshields contact the shell. This can cause problems such as local oxidation or corrosion of the shell, local excedance of its temperature tolerance or local excedance of its tolerance to temperature gradients. Other problems related to direct contact include detrimental changes in the morphology or microstructure of the shell, changes that may be exacerbated by elevated temperatures.